check.h: remove obsolete header

Since we switched from autotools to Meson in commit
94592c1406, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

src/event/WakeFD.hxx

@@ -20,9 +20,7 @@
 #ifndef MPD_WAKE_FD_HXX
 #define MPD_WAKE_FD_HXX
 
-#include "check.h"
-
-#include <assert.h>
+#include "config.h"
 
 #ifdef USE_EVENTFD
 #include "system/EventFD.hxx"
@@ -32,4 +30,4 @@
 #define WakeFD EventPipe
 #endif
 
-#endif /* MAIN_NOTIFY_H */
+#endif